Rigorous monitoring of a large-scale marine stock enhancement program demonstrates the need for comprehensive management of fisheries and nursery habitat

Abstract

Release of captively-bred individuals into the wild is one of the most popular tools in fisheries, forestry, and wildlife management, and introgression of hatchery-reared animals into wild populations is of global concern. However, research and monitoring of impacts on wild populations are generally lacking, and the benefit of hatcheries for long-term fisheries and conservation goals is unclear. Using spatio-temporal genetic monitoring and a four-dacade time series of catch data, we quantified the effects on the size and genetic diversity of wild populations of one of the world’s largest marine stock enhancement programs–the red sea bream ( Pagrus major ) in Kagoshima Bay, Japan. Our analyses found that the stock enhancement program reduced genetic diversity of the population, but the genetic effect diminished with increased size of the wild population. Increases to the seaweed communities and reduced fishing efforts were the primary factors associated with the wild population recovery; effects of aquaculture were much smaller. Our results represent crucial evidence that hatcheries for enhancement and conservation of populations cannot be successful over the long term unless sufficient efforts are also made to reduce harvest rates and rehabilitate natural habitats.

Lessons from Japan marine stock enhancement and sea ranching programmes over 100 years

“…The SCFM approach was first applied to red sea bream in Kagoshima Bay. To identify released fish at this location, almost all red sea bream landed at the Kagoshima Fish Market were checked for a deformity of the internostril epidermis (Shishidou 2002; Shishidou & Kitada 2007; Kitada et al 2019). To sample Japanese flounder in Miyako Bay, all flounder at the Miyako fish market were examined (Okouchi et al 1999, 2004).…”

“…As seen in this example, the hatchery releases of red sea bream into Kagoshima Bay substantially increased fisheries production for the first 15 years, a period during which the programme came to be regarded as representative and successful in Japan; importantly, however, the catch of hatchery fish then steadily decreased and remained very low. The declining catch of red sea bream in Kagoshima Bay was attributed to genetic effects (Kitada et al 2019) through unintended domestication/selection by captive breeding (Ford 2002; Araki et al 2007, 2008). Ample evidence exists that captive breeding of salmonids can reduce the fitness of hatchery salmon in the wild (Reisenbichler & McIntyre 1977; Fleming et al 2000; McGinnity et al 2003; Araki et al 2007; Christie et al 2014).…”

“…Although more areas would thus be affected, the influence on the meta-population was not very strong. A genetic diversity analysis of fish collected from Kagoshima Bay suggested that the genetic effects of hatchery releases were gradually diluted by backcrossing with wild populations (Kitada et al 2019), which would diminish the genetic effects of captive breeding if such effects were additive (Roberge et al 2008). The increasing population of wild fish in Kagoshima Bay showed no fitness decline attributable to the small proportion of hatchery fish in the meta-population.…”

“…Early theoretical work predicted that >99% (50%) of wild genes with additive effects are replaced by hatchery genes in 12 generations (2 generations for 50% of wild genes) in the case of equal fitness between hatchery and wild fish at a stocking rate of 0.5 (Matsuishi et al 1995; see also Fig. 5 in Kitada et al 2019). Taking all of these results in consideration, I speculate that all chum salmon returning to Japan are hatchery-released fish or wild-born hatchery descendants.…”

“…For the SCFM, researchers sampled landings by visiting fish markets to cover the range of the stocking effect. The SCFM approach was first applied to red sea bream in Kagoshima Bay, where almost all red sea bream landed at the Kagoshima Fish Market were checked for deformity of the internostril epidermis to identify released fish (Kitada et al, 2019; Shishidou, 2002; Shishidou & Kitada, 2007). In the case of Japanese flounder in Miyako Bay, all flounder at the Miyako fish market were examined (Okouchi, Kitada, Iwamoto, & Fukunaga, 2004; Okouchi, Kitada, Tsuzaki, Fukunaga, & Iwamoto, 1999).…”